// For flags

CVE-2025-40777

A possible assertion failure when 'stale-answer-client-timeout' is set to '0'

Severity Score

7.5
*CVSS v3.1

Exploit Likelihood

*EPSS

Affected Versions

*CPE

Public Exploits

0
*Multiple Sources

Exploited in Wild

-
*KEV

Decision

Attend
*SSVC
Descriptions

If a `named` caching resolver is configured with `serve-stale-enable` `yes`, and with `stale-answer-client-timeout` set to `0` (the only allowable value other than `disabled`), and if the resolver, in the process of resolving a query, encounters a CNAME chain involving a specific combination of cached or authoritative records, the daemon will abort with an assertion failure.
This issue affects BIND 9 versions 9.20.0 through 9.20.10, 9.21.0 through 9.21.9, and 9.20.9-S1 through 9.20.10-S1.

If a `named` caching resolver is configured with `serve-stale-enable` `yes`, and with `stale-answer-client-timeout` set to `0` (the only allowable value other than `disabled`), and if the resolver, in the process of resolving a query, encounters a CNAME chain involving a specific combination of cached or authoritative records, the daemon will abort with an assertion failure. This issue affects BIND 9 versions 9.20.0 through 9.20.10, 9.21.0 through 9.21.9, and 9.20.9-S1 through 9.20.10-S1.

It was discovered that Bind incorrectly handled configurations where the stale-answer-client-timeout option is set to 0. A remote attacker could possibly use this issue to cause Bind to crash, resulting in a denial of service.

*Credits: N/A
CVSS Scores
Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High
Attack Vector
Network
Attack Complexity
Low
Authentication
None
Confidentiality
None
Integrity
None
Availability
Complete
* Common Vulnerability Scoring System
SSVC
  • Decision:Attend
Exploitation
None
Automatable
Yes
Tech. Impact
Partial
* Organization's Worst-case Scenario
Timeline
  • 2025-04-16 CVE Reserved
  • 2025-07-16 CVE Published
  • 2025-07-18 CVE Updated
  • 2025-07-22 EPSS Updated
  • ---------- Exploited in Wild
  • ---------- KEV Due Date
  • ---------- First Exploit
CWE
  • CWE-617: Reachable Assertion
CAPEC
References (1)
URL Tag Source
URL Date SRC
URL Date SRC
URL Date SRC
https://kb.isc.org/docs/cve-2025-40777 2025-07-16
Affected Vendors, Products, and Versions
Vendor Product Version Other Status
Vendor Product Version Other Status <-- --> Vendor Product Version Other Status
ISC
Search vendor "ISC"
 BIND 9
Search vendor "ISC" for product "BIND 9"
 >= 9.20.0 <= 9.20.10
Search vendor "ISC" for product "BIND 9" and version " >= 9.20.0 <= 9.20.10"
en
Affected
ISC
Search vendor "ISC"
 BIND 9
Search vendor "ISC" for product "BIND 9"
 >= 9.21.0 <= 9.21.9
Search vendor "ISC" for product "BIND 9" and version " >= 9.21.0 <= 9.21.9"
en
Affected